Anybody find these big focusing errors Ctein mentions in his Post Exposure writings? I'm thinking about running the simple test he explains, just for the hell of it, but I'm wondering why nobody else ever talks about this phenomenon, or why even in workshops with some of photography's big names, I've never heard about it? Based on the way Ctein describes it (with focus for an 8x10 print potentially being off by something like 15mm (!!!)), that would be horrible. And it seems like you can't avoid it even with a high quality APO enlarging lens. Strange, and disturbing.

08-01-2011, 12:45 PM

Greg Davis

I read that part, but since I don't own the blue filter for my grain focuser I haven't tested it. Try it and post your results.

08-01-2011, 12:47 PM

BetterSense

Could you please better explain this "gotcha" so that some of us can understand it? It's not very clear from your post.

I also remember someone posting here that they did test it and found just the opposite, that is, they had sharper prints using the blue filter. I believe they also had a blind test from 3rd parties choose the sharpest print, which was the one printed with the blue filter focus. Personally, I think it works for some people and not others due to individual ability to focus blue light with the naked eye.

Greg, there are two LCA issues discussed by Ctein. It seems like you're referring to the Patrick Gainer one where he says we should not focus with blue light. Is this the one you're thinking of?

Here I was referring to the other issue he raises, with VC papers, in which he found depending on how much contrast you dial in (ie how much blue light you let through), the plane of focus can apparently shift by a huge amount compared to using yellow filtration. This would be due to the blue emulsion being significantly sensitive to light around the UV spectrum, and even well corrected lenses are not corrected for longitudinal chromatic abberation outside the visible spectrum.

I wonder if you can solve the entire problem (if there is one) by simply putting a UV filter in the light path.

No not that one. That's the "Gainer effect". Go to page 131. That's where it really gets ugly.

08-01-2011, 01:27 PM

Greg Davis

Yes, I am thinking of the Gainer effect. I'll reread the other and think about it a few minutes.

08-01-2011, 01:45 PM

chimneyfinder

Hello Michael, I recently read Ctein's book, generously offered as a free download (linked from Mike Johnson's 'theonlinephotographer'), and thought that I didn't need to do a whole series of tests regarding the focus anomaly of paper, because it would be very time consumming given that I enlarge 4 different formats to 3 different sizes using half a dozen different papers. Hence, the 'simple' test to work out the paper's plane of focus as opposed to the optical one (yes, that's right folks !) would be a major undertaking to have real meaning. However, out of interest, I will try it for one set up to see if there is a significant difference: not sure when, but it is on my 'to do' list.
Only ever under a magnifier have I noticed that a print hasn't been as sharp as hoped and put this down to the point of capture, and here lies the context in which I put the existence of focus anomaly: so far my prints are sharp enough for normal viewing, so I am not alarmed that they could be sharper. However, I am interested in this and so will, as said, explore.
For anyone browsing this, the research by Ctein outlined in 'Post Exposure: Advanced Techniques for the Photographic Printer' showed focus anomalies on variable contrast paper due to the spectral sensitivity of papers receiving a different plane of focus to that of our eyesight. This differs between lens and paper combinations, but is a recognised phenomena by manufacturers. Basically, there is little in lens or paper manufacture to overcome this. However, as said, this is looking at critical sharpness and under normal viewing most prints are probably judged to be 'sharp enough', if they aren't then read the book.